Electronic equipments including a portable information terminal such as a mobile phone must be reduced in size, and in particular in thickness. Various methods for mounting a semiconductor chip have been proposed for such reduction in size. A particularly useful method among the proposed methods is a method in which two semiconductor chips are mounted on the top surface and the back surface of a wiring board so as to face each other. There are two methods as such a method for mounting the semiconductor chips on the top surface and the back surface of a single wiring plate.
The first method is a method in which a mounting/connecting substrate is provided between a wiring board and a mounting board, and the wiring board is connected to the mounting board through the connecting substrate, enabling a semiconductor chip on the back surface to be mounted without contacting the mounting board.
FIG. 11 is a cross-sectional view of a conventional electronic equipment mounted by such a method, for example, a cross-sectional view of a conventional electronic equipment disclosed in Japanese Laid-Open Publication No. 7-240496. As shown in FIG. 11, an electronic equipment is formed with a semiconductor device mounted on a mounting board 108. The semiconductor device includes semiconductor chips 101a, 101b respectively mounted on a top surface and a back surface of a single wiring board 103. The wiring substrate 103 is bonded to, and electrically connected to, the mounting board 108 through a connecting substrate 106. More specifically, electric connection between electrodes 104 of the wiring board 103 and electrodes 116 of the connecting substrate 106 as well as electric connection between the electrodes 116 of the connecting substrate 106 and electrodes 109 of the mounting board 108 are both implemented with solder bumps 107. The thickness of the semiconductor chip 101b is herein set to a value smaller than the total thickness of the connecting substrate 106 and the solder bump 107 so that connection to the mounting board is not hindered. Note that the wiring substrate 103 is electrically connected to the first and second semiconductor chips 101a, 101b through electrodes 102. Moreover, the respective connections between the wiring board 103 and the first and second semiconductor chips 101a, 101b and the connection between the wiring board 103 and the connecting substrate 106 are sealed with a sealing resin 105.
The second method is a method in which a recess is formed at the back surface of a wiring board, and a semiconductor chip is fittingly mounted in the recess. This method also enables a semiconductor chip on the back surface to be mounted without contacting a mounting board.
FIG. 12 is a cross-sectional view of a conventional electronic equipment mounted by such a method, for example, a cross-sectional view of a conventional electronic equipment disclosed in Japanese Laid-Open Publication No. 10-79405. As shown in FIG. 12, a recess is formed at the back surface of the wiring board, and the semiconductor chip 101b is fittingly mounted in the recess. Mounting the semiconductor chip 101b in the recess enables connection between solder bumps 107 and electrodes of a mounting board (not shown) to be achieved without being disturbed by the semiconductor chip 101b. Note that the wiring substrate 103 is electrically connected to the first and second semiconductor chips 101a, 101b through electrodes 102. Moreover, the respective connections between the wiring board 103 and the first and second semiconductor chips 101a, 101b are sealed with a sealing resin 105.
The first and second methods thus enable the wiring substrate to be mounted on the mounting board so that the semiconductor chip on the back surface does not contact the mounting board.
However, the first method necessitates the use of an expensive connecting substrate, increasing the manufacturing costs of the electronic equipment. Moreover, since the wiring substrate is bonded to the mounting board through the connecting substrate, the total thickness is increased due to insertion of the connecting substrate, hindering reduction in thickness.
The second method necessitates formation of a recess in the wiring board, which requires special man-hour. Accordingly, this method also increases the manufacturing costs of the electronic equipment.
One possible way to mount on the mounting board the wiring substrate having a semiconductor chip mounted on both surfaces thereof without using any connecting substrate and without forming any recess in the wiring substrate is to reduce the thickness of the semiconductor chip 101b as much as possible. In this case, however, reduction in thickness of the semiconductor chip 101b reduces the rigidity, resulting in degraded reliability and the like.
Moreover, a wiring substrate on which fine wirings are formed usually has a larger linear expansion coefficient than a mounting board due to its material, that is, the wiring substrate and the mounting board have different linear expansion coefficients. When the wiring substrate and the mounting board having different linear expansion coefficients are soldered together with heat treatment, the wiring substrate is subjected to greater thermal contraction than the mounting board when cooled to room temperature. Therefore, as shown in FIG. 13, the wiring substrate 103 is warped so as to project toward the mounting board 108. Accordingly, the semiconductor chip 101b mounted on the back surface of the wiring substrate 103 is also warped so as to project toward the mounting board, whereby the semiconductor chip 101b is damaged due to contact with the mounting board 108.
Moreover, it is now assumed that the semiconductor chip 101b and the mounting board 108 have a small gap therebetween. In this case, even if the semiconductor chip 101b does not contact the mounting board 108 when mounted, the mounting board 108 may be subjected to bending or torsional stresses by external pressure and the like generated when the product is in use. This would cause the surface of the semiconductor chip 101b to contact the mounting board 108, damaging the semiconductor chip 101b. 